1. Field
This disclosure relates to microwave circuit modules.
2. Description of the Related Art
A circuit module is an assembly of components that are packaged or mounted together and constitute a functional unit for an electronic system. Examples of circuit modules include printed wiring assemblies and hybrid circuit assemblies. Circuit modules commonly include one or more substrates used to connect and support electronic components. Each substrate typically consists of conductive pathways laminated to one or more non-conductive layers. Substrates may be multi-layered, including multiple layers of substrate materials with conductive pathways on and between the substrate layers. Components and conductive pathways on different substrate layers may be connected by interlayer interconnects such as plated-through holes.
The substrate materials may be selected based on electrical properties such as dielectric constant, loss tangent, and dielectric strength; mechanical properties such as stiffness and thermal conductivity; cost; and other properties. Ideally, the various layers of a multilayer substrate could be selected independently based on desired characteristics for each layer. For example, in microwave circuit modules, a substrate layers supporting stripline components such as couplers and splitters would ideally be a material with low loss and well-controlled dielectric constant, while other substrate layers would ideally be selected for other characteristics such as lowest cost.
However, in many applications, multilayer substrates may be subjected to a wide range of ambient temperatures. In such applications, differences in the coefficient of thermal expansion (CTE) of the substrate layers may cause undesired effects such as deformation of the PWB and/or degradation of interlayer interconnects. Thus designers are generally forced to design circuit modules using a limited number of CTE-matched materials for the substrate layers.
Many of the materials used for substrate layers are composites containing reinforcing fibers such as glass fibers. For such composite materials, the CTE along the axis normal to the surfaces of the substrate may be different from the CTE along axes parallel to the surfaces of the substrate.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having a reference designator with the same least significant digits.